The theodolite, as used in land-surveying, levelling, &c., is well known. But the great theodolite, with its vertical circle and telescope adapted to the observation of the heavenly bodies, as used by nautical astronomers, commonly called an alt-azimuth instrument, is almost an observatory per se. By this alone, within three hours on each side of noon, the longitude, latitude, and magnetic variation of a position may be determined.